Looking back at Drupal 7 is a useless exercise for HHVM since neither Drupal 8 or HHVM are still ripe for mass production deployment. HHVM might be, but Drupal 8 is definitely not ready. The Facebook HHVM team is working on support for Drupal 8 (and the Symfony 2 components) for HHVM: http://hhvm.com/frameworks/

PHP became ubiqutious not because it is the best technically, but because it is the easiest to run without hassle. For HHVM to reach this state it needs to run similarly as the default PHP runtime. Not just the web hosting, but all CLI tools (such as Composer, Drush, etc. utilities).

While HHVM will still radically improve PHP execution, it can be argued on how much it will benefit normal (the 99.99999%) of websites. Probably not a lot, but it could significantly lower the CPU requirements of cloud hosting running thousands of Drupal installations.